Energy Conversion Approaches and Materials for High-Efficiency Photovoltaics Martin A
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INSIGHT | REVIEW ARTICLES PUBLISHED ONLINE: 20 DECEMBER 2016 | DOI: 10.1038/NMAT4676 Energy conversion approaches and materials for high-efficiency photovoltaics Martin A. Green* and Stephen P. Bremner The past five years have seen significant cost reductions in photovoltaics and a correspondingly strong increase in uptake, with photovoltaics now positioned to provide one of the lowest-cost options for future electricity generation. What is becoming clear as the industry develops is that area-related costs, such as costs of encapsulation and field-installation, are increasingly important components of the total costs of photovoltaic electricity generation, with this trend expected to continue. Improved energy-conversion efficiency directly reduces such costs, with increased manufacturing volume likely to drive down the addi- tional costs associated with implementing higher efficiencies. This suggests the industry will evolve beyond the standard single-junction solar cells that currently dominate commercial production, where energy-conversion efficiencies are fundamen- tally constrained by Shockley–Queisser limits to practical values below 30%. This Review assesses the overall prospects for a range of approaches that can potentially exceed these limits, based on ultimate efficiency prospects, material requirements and developmental outlook. 1 eports of the first efficient silicon solar cells in 1954 stimu- open-circuit voltage VOC, the current–voltage curve and the limiting lated calculations of ultimate photovoltaic efficiency2,3 and its efficiency5. The limits for standard single-junction cells and the fac- Rdependence on the semiconductor bandgap (Eg). Calculating tors that influence these are described below. current-generation limits in a short-circuited cell is uncomplicated: each incident photon of energy above Eg is assumed to create one Shockley–Queisser limits. Figure 1b shows limits calculated versus carrier that flows in the external circuit (Fig. 1a). Assigning funda- cell bandgap (or, more correctly, the absorption threshold, which is mental limits to the open-circuit voltage (VOC) is more problematic. often smaller thanE g, when effects such as excitonic and phonon- Initial approaches estimated bounds2,3 using Shockley’s semicon- assisted absorption are considered). Also shown are the highest ductor p–n junction theory4, combined with optimistic extrapola- experimental results6, plotted versus energy absorption threshold. tions of relevant material properties. Since there is no unique way of determining this threshold, it was Shockley and Queisser5 found an elegant solution to this dif- chosen for the plotted data as the photon energy, where experi- ficulty. Noting solar cells must absorb sunlight strongly, they real- mental cell photon-to-electron external quantum efficiency, EQEpe, ized the optimal material properties would be related to those of increases to half-peak value at long wavelengths. ideal black bodies, at least for energies above Eg. Unbiased in the Two crystalline materials, Si and GaAs, have demonstrated effi- dark, good cells would receive and emit largely ambient temperature ciency above 25%, with assorted crystalline, polycrystalline and black-body radiation. Emitted photons of energy above Eg over- thin-film materials demonstrating efficiency clustered around the whelmingly originate from radiative recombination of electrons in 21–22% range. A gap follows to the 10–15% range, where several the semiconductor conduction band with valence-band holes, as other materials can be found. the strongest optical process at such energies. Photons generated Three loss categories cause experimental performance to fall in this way would have various fates, such as being reabsorbed or below SQ limits: (1) collection losses, including photon reflection totally-internally-reflected at surfaces, but the number emitted is or absorption in poorly responding cell regions; (2) non-radia- readily quantified by integrating the ambient temperature black- tive recombination occurring together with radiative, decreas- body photon spectrum above Eg. ing output voltages, particularly for poor-quality materials and With a voltage V across the cell (Fig. 1a), Shockley’s p–n junction interfaces; (3) electrical losses, including cell series- and shunt- theory4 shows that carrier concentration near the junction increases resistances. Losses (1) and (3) depend on cell-design specifics. exponentially with qV/kT (where k is Boltzmann’s constant, T is Loss (2) can be quantified for any experimental cell by calculat- absolute temperature and q is electronic charge, with kT/q, the ‘ther- ing its external radiative efficiency7 (ERE), which is defined as mal voltage’, equal to 26.69257 mV at 25 °C). Intuitively, Shockley the fraction of net recombination when the cell is open-circuited and Queisser realized that, for good cells, similar enhancements that is radiative. (This is closely related to the electron-to-photon throughout active volumes would correspondingly increase recom- EQE when measured as a light-emitting diode (LED)). Because bination rates. Hence, the number of photons emitted by good cells SQ theory assumes 100% ERE, ERE determines how closely an under voltage would be the above-bandgap thermal equilibrium experimental cell approaches the ideal. emission enhanced by this exponential factor. Figure 1c shows cell energy-conversion efficiency versus ERE for a range of photovoltaic materials. For crystalline III–V materials, Single-junction cells ERE can be as high as 32.3% for the record 28.8%-efficient GaAs Calculating Shockley–Queisser (SQ) limits follows simply from the cell. For silicon, ERE is lower (0.1–2.2%) and constrained to values above insight. For limiting performance, all cell recombination will well below 100%, as intrinsic non-radiative Auger recombination be radiative. The current that supports this recombination is equal is stronger than radiative recombination8,9. Spanning the silicon to q times the emitted photon-flux, thus allowing calculation of the ERE values are the polycrystalline chalcogenides, copper indium Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia. *e-mail: [email protected] NATURE MATERIALS | VOL 16 | JANUARY 2017 | www.nature.com/naturematerials 23 ©2016 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. REVIEW ARTICLES | INSIGHT NATURE MATERIALS DOI: 10.1038/NMAT4676 a b 45 Sunlight AM1.5D 46,200 40 SQ concentration limits Light emission 35 30 AM1.5G c-GaAs c-Si 25 c-InP CIGS Perovskite 20 mc-Si CdTe c-GalnP Eciency (%) CIS p 15 CZTSSe Dye Organic n 10 nc-Si a-Si I CZTS CGS 5 c-Ge Reflector V 0 0.4 0.8 1.2 1.6 2.0 2.4 Bandgap (eV) c d 30 Silicon GaAs 25 Diuse CdTe CIGS 20 Half-angle θ 15 CdTe Perovskite a-Si eciency (%) OPV CZTS Energy conversion conversion Energy 10 Diuse Dye Module 5 Direct 0 1 2 –7 –6 –5 –4 –3 –2 –1 10 10 10 10 10 10 10 10 10 10 ERE (%) Figure 1 | Light absorption and emission, SQ limits, experimental energy-conversion efficiencies, radiative efficiencies and sunlight incident angles. a, Light absorption and emission from a solar cell under load. b, SQ energy-conversion efficiency limits under global sunlight (AM1.5G) versus energy absorption threshold (solid line), highest experimental values for various materials (data points) and limits for direct AM1.5D sunlight conversion under maximal concentration of 46,200 suns (dashed line), all at a cell temperature of 25 °C. Limits for other concentration ratios lie roughly geometrically intermediate between the solid and dashed lines, such that the curve for a concentration of 215 suns would lie around halfway between the two. c, Energy- conversion efficiency versus ERE for various experimental cells.d , Angular effects in solar energy conversion, showing direct and diffuse solar components, minimum acceptance angle and ground reflection effects. Data taken from refs 6,7. gallium selenide (CIGS) and recently emerging perovskites. Next Ross13, extended by Würfel14, and recently discussed analytically15. come other chalcogenides, CdTe and CZTSS (Cu2ZnSnSySe4–y), The second refinement relates light emission from practical cells to followed by dye-sensitized amorphous silicon and organic pho- black-body emission16. In one of the semiconductor device field’s tovoltaic (OPV) cells. Organic LEDs have high values of electron- surprising reciprocal relationships, emission from an actual cell, to-photon EQE because they are designed differently to their OPV with some provisos, equals that from the black-body ideal at each 10 counterparts , with LED-like design perhaps explaining one out- wavelength and voltage, except multiplied by EQEpe at that wave- lier in Fig. 1c — an organic photovoltaic cell with modest conver- length when used as a solar cell16. (The same relationship also applies sion efficiency despite reasonable ERE. The other outlier, a CdTe to LEDs, although this is possibly not as well-known or exploited in cell with 21% efficiency despite low ERE, is explained by high-qual- that field.) ity processing, which gives a good fill-factor and photogenerated carrier collection. Concentrated sunlight. One way to exceed original SQ limits is to The original SQ theory has been extended by two refinements. focus sunlight. The current in a short-circuited cell increases lin- One is to include stimulated emission, which allows accurate treat- early with photon flux, whereas VOC increases